Viscous soluble fiber derived from psyllium and oat-bran lowers LDL cholesterol levels. In order to test the hypothesis that one mechanism for this effect is reduction of cholesterol absorption we measured the efficiency of intestinal cholesterol absorption on two occasions in normal subjects using a test breakfast with or without added soluble fiber. Percent cholesterol absorption was determined by feeding 30 mg pentadeuterated cholesterol in oil with the meal and simultaneously injecting intravenously 15 mg hexadeuterated cholesterol solubilized in Intralipid. The ratio of the two cholesterol tracers in plasma 4 days later was determined by gas chromatography/negative ion mass spectrometry. Two sries of studies were performed, the first comparing soluble psyllium fiber with insoluble cellulose and the second comparing high-fiber oat bran with low-fiber refined wheat. In 8 subjects control cholesterol absorption in the presence of insoluble cellulose was 53.5( 3.2% comp aredto 55.2(2.7% with 5.1 g psyllium fiber (difference not significant). In another 7 subjects cholesterol absorption during control ingestion of low-fiber refined wheat was 47.9(2.0% whereas substitution of high-fiber oat bran (3 g soluble fiber) resulted in 49.3(2.6% absorption (difference not significant). These results suggest that psyllium and oat bran fiber do not reduce the efficiency of cholesterol absorption as a primary mechanism of action in normal subjects.